These sensors use an array of pixels in rows and in columns, with a photosensitive element and several transistors in each pixel.
One advantageous composition of the pixel is a composition such as shown in FIG. 1, with:                a self-biased (or ‘pinned’) photodiode PH,        a charge storage node ND, which is the equivalent of a capacitor,        a transfer transistor T1 for isolating the photodiode from the storage node or, on the contrary, allowing a transfer of charges from the photodiode to the storage node with a view to measuring the potential of the storage node after this transfer,        a transistor T2 for reading charges, configured in voltage follower mode, having its gate connected to the storage node in order to allow the potential of the storage node to be transferred onto the source of the transistor,        a transistor T3 for resetting the storage node, allowing the potential of the storage node to be set at a reference value for measuring this reference potential, with a view to a differential measurement of the potentials of the storage node in the presence of charges coming from the photodiode and in the absence of charges coming from the photodiode;        a pixel selection transistor T4, controlled by a row addressing conductor SEL, allowing the potential of the source of the read transistor T2 to be transferred onto a column conductor COL; the row conductor SEL is common to all the pixels of the same row of pixels; the column conductor COL is common to all the pixels of the same column of pixels,        lastly, optionally, an additional transistor T5 which can have one and/or the other of the two following functions: evacuate the excess electrical charges in the photodiode towards an anti-blooming drain in the case of too intense an illumination, or else reset the potential of the photodiode by completely emptying the accumulated charges towards a drain in order for it to recover its empty potential prior to beginning a new period of integration; the transistor T5 is optional and allows a start of integration time common to all the pixels to be defined.        
It will be noted that the term “transistor” is used in order to facilitate the understanding in terms of electrical circuit diagram such as the diagram in FIG. 1. However, in the physical composition of the pixel, these transistors are not necessarily formed in a conventional manner, independently of the other elements of the pixel, with a source region, a drain region, a channel region separating the source from the drain, and an isolated gate on top of the channel. In the real physical composition of the pixel, certain transistors are in reality formed essentially by an isolated gate to which a control potential can be applied. Thus, for example, the transfer transistor T1 is formed by a transfer gate isolated from the substrate, lying over a region which is situated between the photodiode PH and a diffusion of the N+ type forming the charge storage node; the source of the transistor T1 is the photodiode; the drain of the transistor is the charge storage node. Similarly, the transistor T5 is formed using an isolated gate, adjacent on one side to the photodiode (forming the source of the transistor), and on the other to a charge evacuation drain (forming the drain of the transistor).
In the sensors of the prior art, the existence of what is generally referred to as a dark current, in other words a flow of stray charges even in the absence of illumination, whereas the absence of illumination should lead to an absence of charges. These stray charges reduce the signal/noise ratio and are particularly detrimental when low lighting conditions are measured.
A part of the dark current is due to the interface defects between the region N of the photodiode and an insulating silicon oxide which surrounds the photodiode in order to insulate it from the neighbouring photodiodes. This insulating oxide, generally known by the acronym STI (for “Shallow Trench Isolation”) is contained within a surface trench which surrounds the whole of the photodiode apart from the passage that needs to be reserved for transferring the charges from the photodiode (transistor T1) to the storage node and except for the passage that may need to be reserved for emptying the charges from the photodiode during a reset operation (transistor T5). The interface defects trap electrons; these electrons are subsequently liberated and attracted towards the storage node at the moment of the transfer of charges from the photodiode to the storage node; for this reason they will be considered as useful charges resulting from the illumination whereas they are in reality stray charges not resulting from the illumination.
In order to limit this effect as far as possible, the idea of interposing a region of type P between the photodiode region (of type N) and the trench filled with insulator has already been proposed. This P region is in contact with the active layer of silicon of type P in which the photodiode is formed. It surrounds the N region of the photodiode and prevents the latter from being in direct contact with the isolation trench. It serves as a passivation layer which reacts with the isolation trench so as to neutralize the stray charges. However, in an industrial process, it is not possible to place a region of type P under the gates (which are made of polycrystalline silicon) of the transfer transistor T1 or of the transistor T5 when it is present; the reason for this is that the implantation of type P must in practice be carried out after the formation of the gates and that it is therefore not possible to implant an impurity of type P under the gates; the isolation trenches made of silicon oxide extend however under the gates and it would have been preferable to passivate them here by a P region as elsewhere.
As a consequence, the semiconductor regions under the gates of the transistors T1 and T5 remain directly in contact with silicon oxide and are likely to generate a detrimental dark current, on the principle of what has been presented hereinabove.
The gate of the transfer transistor T1 could be maintained at a slightly negative potential (around −0.7 volts) instead of a zero potential during the integration of photogenerated charges. This voltage would allow holes to be attracted and accumulated under the gate; the electrons trapped at the oxide/silicon interface then recombine with these mobile holes and disappear, thus eliminating the risk that undesirable electrons go towards the storage node during the transfer of charges from the photodiode to the storage node.
However, in order to produce this negative voltage, whereas the integrated circuit is powered between 0 volts and a positive voltage of 3 to 5 volts, a charge pump must be used. Charge pumps have a low efficiency, lower than 50% and sometimes even lower than 30%. They therefore consume a current much higher than the current that they have to supply.
In one mode of operation of the sensor referred to as “global shutter” mode, in other words with a global exposure time, the transfer phase is simultaneous for all the pixels of the sensor. All the transfer gates of the pixel array are activated at the same time for transferring the charges from the photodiodes into the storage nodes. For a sensor with 2 million pixels, the current demand in the charge pump in order to return to a potential of −0.7 volts at the moment when the transfer pulse ends may exceed 80 milliamps, and this is with the proviso that the control pulse fall time is forced to at least 300 nanoseconds so that the transition is not too abrupt. The 80 milliamps supplied by the charge pump for discharging the equivalent capacitance of all the transfer gates connected in parallel can correspond to an overall current consumption of 250 milliamps owing to the low efficiency of the charge pump.